module Opaleye.Exists (exists) where

import           Opaleye.Field (Field)
import           Opaleye.Internal.Column (Field_(Column))
import           Opaleye.Internal.QueryArr (runSimpleQueryArr, productQueryArr)
import           Opaleye.Internal.PackMap (run, extractAttr)
import           Opaleye.Internal.PrimQuery (PrimQuery' (Exists))
import           Opaleye.Internal.Tag (next)
import           Opaleye.Select (Select)
import           Opaleye.SqlTypes (SqlBool)

-- | True if any rows are returned by the given query, false otherwise.
--
-- This operation is equivalent to Postgres's @EXISTS@ operator.
exists :: Select a -> Select (Field SqlBool)
exists q = productQueryArr (f . runSimpleQueryArr q)
  where
    f (_, query, tag) = (Column result, Exists binding query, tag')
      where
        (result, [(binding, ())]) = run (extractAttr "exists" tag ())
        tag' = next tag